The libevent README.md states that EVENT_STAGE_VERSION defaults to beta when building using cmake.
# set EVENT_STAGE_VERSION
EVENT__STAGE_VERSION:STRING=beta
I did not change this variable when building, so beta appeared in the version string in the generated event-config.h.
I verified that the libevent C files and non-auto-generated header files are identical between github release-2.1.8-stable tag and libevent-2.1.8-stable.tar.gz on the libevent website.
It’s not possible to build on Visual Studio using libevent-2.1.8-stable.tar.gz because the required cmake files (CMakeLists.txt, …) do not exist in this tarball.
In summary, you should build with libevent 2.1.8-stable.